This invention relates to the field of acoustic damping. More particularly, the present invention is an inflatable elastic membrane that, when inflated, acts as an acoustic resonator to damp acoustic resonances in an enclosed volume, such as that contained by the fairing of a launch vehicle.
Traditional acoustic resonators are fabricated from a rigid material, and enclose a vented volume. A neck usually controls the length of the vent. A well known example of such an acoustic resonator is a wine bottle. An acoustic resonator can be used to reduce acoustic transmission into an enclosed space, such as that of a launch vehicle or an airplane. If the unwanted acoustics are tonal, i.e., occurring at a single frequency or within a narrow bandwidth, the resonator is configured as a reactive device with low damping. Introducing the reactive resonator significantly increases the reactive impedance over a narrow frequency band.
By tuning the resonator, the narrow frequency band may be made to coincide with the tonal disturbance. The disturbance is then partially or completely reflected away from the acoustic space, and its transmission into the space is decreased. If the unwanted acoustics are broadband and the acoustic space has a low number of lightly damped resonances, the resonator may be configured as a resistive device with high damping. Introducing a resistive or damped resonator into the acoustic space adds damping to the acoustic resonances. Although all lightly damped acoustic resonances will tend to increase their damping level by introducing a damped resonator, the largest increase in damping will result at the acoustic resonance to which the resonator is tuned.
Multiple resonators may be used simultaneously to attenuate multiple tones or multiple band widths. In either case, tuning is achieved by varying the volume and/or neck length of a traditional resonator. This tuning process is well understood and has been described in a number of textbooks, for example, Philip M. Morse & K. Uno Ingard, Theoretical Acoustics 489-490 (Princeton University Press, 1986), and Leo L. Beranek, Noise and Vibration Control 370-376 (2d ed., Institute of Noise Control Engineering, 1988).
The beneficial effect of the resonator is compromised by the volume it occupies. Furthermore, the use of an acoustic resonator is limited by its weight, especially when multiple resonators are required. Prior art acoustic resonators are described in the following patents:
U.S. Pat. No. 6,493,180, “Hard Disk Drive Cover That Contains a Helmholtz Resonator Which Attenuates Acoustic Energy,” issued Dec. 10, 2002; U.S. Pat. No. 6,485,205, “Media Weight Sensor Using an Acoustic Resonator,” issued Nov. 26, 2002; U.S. Pat. No. 6,151,396, “Active Acoustic Resonator for Abating Noise,” issued Nov. 21, 2000; and U.S. Pat. No. 6,116,375, “Acoustic Resonator,” issued Sep. 12, 2000.
There is a need in the art for a lightweight, compact resonator that can be tuned to damp low frequency acoustic resonances occurring in an enclosed volume, as well as provide broadband reduction for higher frequency resonances. The present invention fulfills this need in the art.